53-Qubit Quantum Simulator Observes Quantum Magnetism in Action

Photonics.comJan 2018
COLLEGE PARK, Md., Jan. 2, 2018 — Two independent teams of scientists have used more than 50 interacting atomic quantum bits (qubits) to mimic magnetic quantum matter, setting what they believe to be a new record for quantum simulation. In recent years, scientists have controlled 10 to 20 atomic qubits in small-scale quantum simulations.

As the basis for its quantum simulation, a research team from the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST) used up to 53 individual ytterbium ions, i.e., charged atoms trapped in place by gold-coated and razor-sharp electrodes. A complementary design by Harvard and MIT researchers used 51 uncharged rubidium atoms confined by an array of laser beams.

Strings of atomic qubits are used to probe quantum magnetism. Each row of bright lights and dark spots is a fluorescence snapshot of the atom string. Physicists use lasers to measure the qubits during the quantum simulation. The result, either dark or bright, allows them to extract information about the magnetic state of the system. Courtesy of J. Zhang et al. and E. Edwards.Scientists in industry and academia are working to build prototype quantum computers that can control an ever greater number of qubits. One challenge they face is that qubits are delicate and must stay isolated from the environment to protect the device’s quantum nature. With each added qubit, this protection becomes more difficult, especially if qubits are not identical from the start, as is the case with fabricated circuits.

This is one reason why atoms are being used to simplify the process of scaling up to large-scale quantum machinery.

Unlike the integrated circuitry of modern computers, atomic qubits reside inside of a room-temperature vacuum chamber that maintains a pressure similar to outer space. This isolation is necessary to keep the destructive environment at bay and allows scientists to precisely control the atomic qubits with a highly engineered network of lasers, lenses, mirrors, optical fibers and electrical circuitry.

“Each ion qubit is a stable atomic clock that can be perfectly replicated. They are effectively wired together with external laser beams. This means that the same device can be reprogrammed and reconfigured, from the outside, to adapt to any type of quantum simulation or future quantum computer application that comes up,” said professor Christopher Monroe.

In the 53-qubit simulator, the ion qubits are made from atoms that all have the same electrical charge and therefore repel one another. As they push each other away, an electric field generated by a trap forces them back together. The two effects balance each other, and the ions line up single file. Researchers leverage the inherent repulsion to create deliberate ion-to-ion interactions, which are necessary for simulating the interaction of quantum matter.

The quantum simulation begins with a laser pulse that commands all the qubits into the same state. Then, a second set of laser beams interacts with the ion qubits, forcing them to act like tiny magnets, each with a north and a south pole. This second step occurs suddenly, jarring the qubits into action.

As magnets, the qubits can either align their poles with their neighbors to form a ferromagnet; or they can point in random directions, yielding no magnetization. The researchers can change the relative strengths of the laser beams and observe which phase of quantum matter “wins out” under different laser conditions.

The entire simulation takes only a few milliseconds. By repeating the process many times and measuring the resulting states at different points during the simulation, the team can see the process as it unfolds from start to finish. The researchers observe how the qubit magnets organize as different phases form — dynamics that they say are nearly impossible to calculate using conventional means, because there are so many interactions.

“What makes this problem hard is that each magnet interacts with all the other magnets,” said researcher Zhexuan Gong. “With the 53 interacting quantum magnets in this experiment, there are over a quadrillion possible magnet configurations, and this number doubles with each additional magnet. Simulating this large-scale problem on a conventional computer is extremely challenging, if at all possible.”

This is an artist's depiction of a quantum simulation. Lasers manipulate an array of more than 50 atomic qubits to study the dynamics of quantum magnetism. Courtesy of E. Edwards/JQI.This quantum simulator is suitable for probing magnetic matter and related problems. Other kinds of calculations may need a more general quantum computer with arbitrarily programmable interactions in order to get a boost.

“Quantum simulations are widely believed to be one of the first useful applications of quantum computers. After perfecting these quantum simulators, we can then implement quantum circuits and eventually quantum-connect many such ion chains together to build a full-scale quantum computer with a much wider domain of applications,” said professor Alexey Gorshkov.

As they look to add even more qubits, the team believes that its simulator could be used in more computationally challenging terrain, beyond magnetism.

“We are continuing to refine our system, and we think that soon, we will be able to control 100 ion qubits or more,” said researcher Jiehang Zhang. “At that point, we can potentially explore difficult problems in quantum chemistry or materials design.”